The present invention relates to an NAND flash memory device. More particularly, the present invention relates to a method of measuring a channel boosting voltage for the purpose of estimating a program disturbance characteristic of a NAND flash memory device.
An NAND flash memory device includes a plurality of cell blocks.
The cell block has cell strings 101 and 102 where a plurality of cells for storing data are connected in series, a drain select transistor 110 connected between the cell strings 101, 102 and a bit line BL, and a source select transistor 120 connected between the cell strings 101, 102 and a common source line CSL. Here, the number of the cell strings 101 and 102 is identical to that of the bit lines BL. Accordingly, the number of each of the drain select transistors 110 and the source select transistor 120 is identical to that of the cell stings 101 and 102.
A first bias voltage is applied to a gate of a cell through a word line WL so as to operate the cell, and a second bias voltage is provided to a drain of the drain select transistor 110 through the bit line BL. In addition, a third bias voltage is applied to a source of the source select transistor 120 through the common source line CSL.
On the other hand, in the cell of the NAND flash memory device, a gate where a tunnel oxidation film, a floating gate, a dielectric film and a control gate are laminated is formed on a given area of areas of a semiconductor substrate. Additionally, a bonding section is formed on a first part of the semiconductor substrate. Here, the first part corresponds to a lower part of both sides of the gate.
To program or erase the cell, the above NAND flash memory device injects electrons in the floating gate of the cell using a FN tunneling or ejects electrons from the floating gate using the FN tunneling. Here, the erase is performed on a unit of block, and the program is performed on a selected cell.
To program a selected cell M11 in the NAND flash memory device, a program voltage of about 18V is applied to a selected word line Selected WL, and a pass voltage of about 8V is provided to a word line not selected Pass WL. Further, a ground voltage Vss is applied to a selected bit line Selected BL, and a power supply voltage Vcc is provided to a bit line not selected Unselected BL. Here, the power supply voltage Vcc is applied to a drain select line DSL, and the ground voltage Vss is provided to the source select line SSL. Moreover, the power supply voltage Vcc is applied to the common source line CSL, and the ground voltage Vss is provided to a well (bulk).
In this case, the program voltage is applied to a control gate of a cell M14 related to the bit line not selected Unselected BL. However, a channel is boosted with a voltage rate corresponding to the coupling between the program voltage, the pass voltage and a precharge voltage provided from the bit line BL.
A voltage which the boosted channel has, i.e. channel boosting voltage prevents the FN tunneling of cells related to the bit line not selected Unselected BL, thereby preventing a program disturbance.
On the other hand, a cell to which the pass voltage is applied may be programmed. This is referred to as pass disturbance. Here, the cell is one of cells related to the selected bit line Selected BL.
In FIG. 1, the cell M11 is a cell to be programmed, the cells M12 and M13 are pass disturbance cells, and the cell M14 is a program disturbance cell.
It is important to secure program disturbance characteristics in the NAND flash memory device for the development of a product.
Factors such as a program voltage, a pass voltage, a threshold voltage of a drain select transistor, a leakage current GIDL (channel leakage current), a channel capacitance, a program time, number of a program NOP, etc. affect the program disturbance characteristics.
In addition, the channel boosting voltage is determined depending on the factors. Accordingly, the program disturbance characteristics may be estimated in case that the channel boosting voltage is measured.
However, a method of measuring the channel boosting voltage does not exist in the conventional art. This is because the channel is changed from a floating state into a ground state when it is examined (or probed) in order to measure the channel boosting voltage, and so the channel boosting voltage is changed in accordance with the converted state.
That is, since the channel boosting voltage is discharged through a probe in case of connecting a probe tip so as to measure the channel boosting voltage, the channel boosting voltage is not measured until a failed bit is found in a product. Particularly, it is verified that a cell where the channel boosting voltage is reduced by a certain leakage current is existed in the NAND flash memory device only when the failed bit is found in the product. Hence, the optimization of a process may be not timely achieved.
Accordingly, the present art measures the channel boosting voltage through a simulation, and so it is difficult to predict the channel boosting voltage changed by the leakage current of the channel.